The present invention relates to dc-to-dc power converters and, more particularly, to a control method and apparatus for a high voltage, reduced size converter.
Voltage converters are commonly used to convert direct current (dc) voltage at one level to voltage at another level. In many instances, the voltage converter is used to step up a relatively low voltage, e.g., 115-230 volts, to a relatively high voltage, e.g., several thousand volts. By way of example, a voltage converter for x-ray diagnostic machines may use rectified utility line voltage at about 115 volts to produce an x-ray tube grid voltage of about 3600 volts. An exemplary converter circuit for such purpose includes a rectifier to convert 115 volt ac voltage to dc voltage, a voltage doubler to raise the dc voltage to about 320 volts and a half-bridge dc-to-dc converter to raise the 320 volts to 3600 volts. The half-bridge converter typically uses a series pair of switching devices to connect a resonant capacitor and a primary winding of a transformer to the 320 volt source. This comprises a series resonant circuit, where the resonant inductor is the transformer leakage inductance. The transformer secondary winding is coupled to an output rectifier. One of the disadvantages of this type of converter using single resonant pulse control to transfer sufficient energy to the output capacitor to produce the regulated 3600 volt dc output is the large transformer magnetic core flux which increases the weight, volume, and cost of the converter circuit.